Since I first started shipping mantids I have only given a live arrival guarantee on next day air shipments when temperatures were within the noted acceptable range. However, I pretty much always replace priority shipments as well. So, as there really is no point to force people to pay extra for fear the nymphs will not arrive alive, I have changed live arrival guarantee to all shipments that fall within the accepted temperature range. Next day is still the best idea if you prefer the nymphs ASAP or temps fall outside of those ranges. Cold packs do not last more than a day or so, and heat packs generally for a few days. I tend to have more issues in hot weather than cold. So if your temps will be above 90 I still recommend next day, even though most of the time they are still just fine. I am not as concerned with cold weather.